IO

FGJ: Create:2024/01/13 Update: (2024-10-24)

Structure #
FileDescriptor #
文件描述符类的实例充当底层机器特定结构的不透明句柄，表示打开的文件、打开的套接字或另一个字节源或接收器。文件描述符的主要实际用途是创建FileInputStream或FileOutputStream来包含它。

它相当于将指定文件的信息与新建的文件指针fp相关联，在 FILE 结构内部记录了这样一些信息：文件内部的当前读写位置、读写报错的记录、文件结尾指示器、缓冲区开始位置的指针、文件标识符、一个计数器（统计拷贝进缓冲区的字节数）等等。后继的操作就可以使用这个指针（而不是文件名）来处理指定文件。参考

InputStream #

signature： public abstract class InputStream implements Closeable {}

FileInputStream #

摘抄代码 #

jdk代码来自【openjdk 23-internal】

package java.io;

import java.nio.channels.FileChannel;
import sun.nio.ch.FileChannelImpl;

public
class FileInputStream extends InputStream
{
    /* File Descriptor - handle to the open file */
    private final FileDescriptor fd;

    /**
    * The path of the referenced file
    * (null if the stream is created with a file descriptor)
    */
    private final String path;

    public FileInputStream(String name) throws FileNotFoundException {
        this(name != null ? new File(name) : null);
    }

    public FileInputStream(File file) throws FileNotFoundException {
        String name = (file != null ? file.getPath() : null);
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkRead(name);
        }
        if (name == null) {
            throw new NullPointerException();
        }
        if (file.isInvalid()) {
            throw new FileNotFoundException("Invalid file path");
        }
        fd = new FileDescriptor();
        fd.attach(this);
        path = name;
        open(name);
    }

    private native void open0(String name) throws FileNotFoundException;

    private void open(String name) throws FileNotFoundException {
        open0(name);
    }

    private static native void initIDs();

    private native void close0() throws IOException;

    static {
        initIDs();
    }
}

jfieldID fis_fd; /* id for jobject 'fd' in java.io.FileInputStream */

/**************************************************************
* static methods to store field ID's in initializers
*/

JNIEXPORT void JNICALL
Java_java_io_FileInputStream_initIDs(JNIEnv *env, jclass fdClass) {
    fis_fd = (*env)->GetFieldID(env, fdClass, "fd", "Ljava/io/FileDescriptor;");
}

/**************************************************************
* Input stream
*/

JNIEXPORT void JNICALL
Java_java_io_FileInputStream_open0(JNIEnv *env, jobject this, jstring path) {
    fileOpen(env, this, path, fis_fd, O_RDONLY);
}

JNIEXPORT jint JNICALL
Java_java_io_FileInputStream_read0(JNIEnv *env, jobject this) {
    return readSingle(env, this, fis_fd);
}

void
fileOpen(JNIEnv *env, jobject this, jstring path, jfieldID fid, int flags)
{
    WITH_PLATFORM_STRING(env, path, ps) {
        FD fd;

#if defined(__linux__) || defined(_ALLBSD_SOURCE)
        /* Remove trailing slashes, since the kernel won't */
        char *p = (char *)ps + strlen(ps) - 1;
        while ((p > ps) && (*p == '/'))
            *p-- = '\0';
#endif
        fd = handleOpen(ps, flags, 0666);
        if (fd != -1) {
            jobject fdobj;
            jboolean append;
            fdobj = (*env)->GetObjectField(env, this, fid);
            if (fdobj != NULL) {
                // Set FD
                (*env)->SetIntField(env, fdobj, IO_fd_fdID, fd);
                append = (flags & O_APPEND) == 0 ? JNI_FALSE : JNI_TRUE;
                (*env)->SetBooleanField(env, fdobj, IO_append_fdID, append);
            }
        } else {
            throwFileNotFoundException(env, path);
        }
    } END_PLATFORM_STRING(env, ps);
}

/* field id for jint 'fd' in java.io.FileDescriptor */
jfieldID IO_fd_fdID;

构造器初始化解读 #
1. 调用静态方法initIDs()用来初始化FileDescriptor fd属性，此处并不是赋值，而是让FileInputStream.c 中的fis_fd保存属性位置。方便后续通过C代码能够获取当前fd属性对象。近而对当前FileInputStream中的fd对象中的属性fd赋值真正的文件描述符id，例如整型值23.
2. 通过open0()方法调用原生方法获取1中需要的文件描述符id，
  2.1 将fis_fd通过fileOpen函数传入io_util_md.c中.
  2.2 fd = handleOpen(ps, flags, 0666);尝试打开给定路径的文件。获取文件描述符id。
3. 将2中获取的文件描述符id赋值给FileInputStream中的fd对象中的属性fd
  3.1 fdobj = (*env)->GetObjectField(env, this, fid); 获取当前FileInputStream对象中的FileDescriptor fd这个对象。
  3.2 (*env)->SetIntField(env, fdobj, IO_fd_fdID, fd); IO_fd_fdID 定义在FileDescriptor_md.c文件中。对3.1中的对象fdobj赋值 fd.
4. 后续原生读写都会通过这个FD进行。

📓 Archive

IO

Structure #

FileDescriptor #

InputStream #

FileInputStream #

摘抄代码 #

构造器初始化解读 #

Reference #